Satellite investigation of tropical cirrus lifecycle and relation to water vapor and deep convection

Tropical cirrus lifecycle and relation to deep convection are examined by analyzing satellite derived cloud data and NCEP/NCAR reanalysis wind field. Also discussed is the relationship between tropical cirrus and upper tropospheric water vapor (UTWV). The analysis in this study is conducted in a Lagrangian framework, which assumes cirrus, like other tracers, drift with the upper tropospheric wind, while at the same time, going through their lifecycle from formation to maturation to decay. The Lagrangian trajectory analysis shows that the decay of deep convection is immediately followed by the growth of cirrostratus and then the decay of cirrostratus is followed by the growth of cirrus. Cirrus properties continuously evolve along the trajectories as they gradually thin out and move to the lower levels. Typical tropical cirrus systems last for 19 - 30 hours, which is much longer than cirrus particle lifetimes. Consequently, tropical cirrus systems can advect over large distances, about 600 - 1000 km, during their lifetimes. Stronger convective systems tend to be associated with cirrus having longer lifetime. Based on their relationship to convective systems, detrainment cirrus are distinguished from in situ cirrus. The interaction between cirrus and UTWV is explored by comparing the evolution of UTWV along composite clear trajectories and trajectories with cirrus. Cirrus are found to be associated with an upward transport of water vapor that moistens the upper troposphere. The maintenance of UTWV is discussed by using a simple, conceptual model.